Energy storage is vitally important for the stability of the electric grid as it relies more on “non-dispatchable” sources of power such as solar and wind. Additionally, there are many applications where long term power may be needed apart from any grid such as in military or emergency situations. Currently, the most common solutions are lead-acid batteries and diesel generators. The shortcomings of these systems are well known. Most electrochemical batteries show a significant degradation in performance as they are discharged. This is evidenced by reduced voltage output and reduced maximum currents observed during the discharge of the battery. Batteries also tend to be less efficient and have reduced service lives when run at greater depths of discharge.
There is increased interest in using stored mechanical energy to ultimately serve as electrical energy storage for grid type applications. This interest is based in part on the robust nature and highly developed state of the art of mechanical power generation equipment, which is economical and scalable. In general, most energy storage schemes compress a working fluid to high pressures or liquefy the working fluid by compression and/or temperature reduction. Alternately, a series of heat pumps (often coupled to generators and/or other heat sources) indirectly accomplish this same goal where working fluid(s) is/are cycled with the combination of heat pumps. This alternative has higher cost associated with extra machinery. In general, to get higher recoveries (efficiencies that we will refer to as “round trip efficiency” or “RTE”) requires more “stages” to recover energy, and each additional stage is met with reduced energy recovery. Thus, these systems require increased capital to achieve high RTE after more than two stages.
Other conventional energy storage systems store energy by compressing a gas to high pressure. Depending on the physical properties (critical temperature) of the gas, it may liquefy. During this compression process, heat is generated and much of this heat is generally lost, which results in reduced RTEs. This is a common problem in compressed air energy storage (CAES). The RTE of conventional systems is around 50%. RTEs of these systems are limited due to the use of assisting gas turbines during operation to increase the overall efficiency of that device. Assisting gas turbines are not a true energy storage systems, but act as a hybrid, wherein stored energy is only recovered if it is also desired to burn natural gas to run the gas turbine to create energy.
Traditional efforts to recover this heat from compression involve storing the heat of compression in a medium (such as water), as sensible heat, which simply raises the temperature of the water. This warm water is stored in an insulated tank until the energy is required to heat the working fluid while expanding it to recover that energy. However, warm water has reduced energy dense medium. Furthermore, the low temperature gradients present engineering challenges to achieve high power output that requires high rates of heat transfer over a low temperature gradient. Problems also exist due to heat from the stored water escaping from the insulated tank giving the system an apparent “self-discharge”, analogous to electrochemical batteries.